Objectives: Location of the primary tumor has prognostic value and predicts the effect of certain therapeutics in synchronous metastatic colorectal cancer. We investigated whether the association between primary tumor resection (PTR) and overall survival (OS) also depends on tumor location.Methods: Data on synchronous metastatic colorectal cancer patients from the Netherlands Cancer Registry (n = 16,106) and Surveillance, Epidemiology, and End Results (SEER) registry (n = 19,584) were extracted. Cox models using time-varying covariates were implemented. Median OS for right-sided colon cancer (RCC), left-sided colon cancer, and rectal cancer was calculated using inverse probability weighting and a landmark point of 6 months after diagnosis as reference.Results: The association between PTR and OS was dependent on tumor location (P < 0.05), with a higher median OS of upfront PTR versus upfront systemic therapy in Netherlands Cancer Registry (NCR) of 1.9 (95% confidence interval: 0.9-2.8), 4.3 (3.3-5.6), and 3.4 (0.6-7.6) months in RCC, left-sided colon cancer and rectal cancer, respectively. In SEER data, the difference was 6.

We studied the prognostic value of primary tumor sidedness in metastatic colorectal cancer over time and across treatment lines. Population data on synchronous metastatic colorectal cancer patients were extracted from the Netherlands Cancer Registry and SEER database. Pubmed, EMBASE and Cochrane library were searched for prospective studies on metastatic colorectal cancer to conduct a meta‐analysis. Inclusion criteria consisted of metastatic disease, systemic treatment with palliative intent and specification of primary tumor location. Data were pooled using a random‐effects model. For the population‐based data, multivariable Cox models were constructed. The Grambsch‐Therneau test was conducted to evaluate the potential time‐varying nature of sidedness. Meta‐regression incorporating treatment‐line as variable was conducted to test the pre‐specified hypothesis that the prognostic value of sidedness varies over time. Analysis of 12 885 and 16 160 synchronous metastatic colorectal cancer patients registered in the Netherlands Cancer Registry and SEER database, respectively, indicated a time‐varying prognostic value of sidedness (P < .01). Thirty‐one studies were selected for the meta‐analysis (9558 patients for overall survival analysis). Pooled univariable hazard ratioleft‐sided/right‐sided for overall survival was 0.71 (95% CI: 0.65‐0.76) in 1st‐line, 0.76 (0.54‐1.06) in 2nd‐line and 1.01 (0.86‐1.19) in 3rd‐line studies. Hazard ratios were significantly influenced by treatment line (P = .035). The prognostic value of sidedness of the primary tumor in metastatic colorectal cancer patients treated with palliative systemic therapy decreases over time since diagnosis, suggesting that sidedness may not be a useful stratification factor in late‐line trials. This decrease in prognostic value should be taken into account when providing prognostic information to patients.